512 REPORT OF NATIONAL MUSEUM, 1890. 



(3) Physical and chemical properties of rocks. 



Under this head are here grouped several small series of rocks de- 

 signed to show (1) the structure of rocks — the manner in which their 

 various constituent parts are associated or grouped together to form 

 rock masses ; (2) the specific gravity of rocks ; (3) the chemical compo- 

 sition ; (4) the color, and (5) the fracture and manner of breaking. 



I. — The structure of rocks. 



A. Macroscopic structure. — It is the object of this exhibit to show all 

 the more typical forms of rock and structure. In other words, to illus- 

 trate by means of specimens the meanings of certain words and phrases 

 in common use in geologic nomenclature, but whose exact significance 

 or force is poorly comprehended by the public at large. In this collec- 

 tion the rocks are divided into four primary groups. That is, they rep- 

 resent four primary types of structure, each of which in its turn exhibits 

 a more or less parallel series of secondary siructures. These types of 

 structure are (1) crystalline ; (2) vitreous or glassy ; (3) colloidal, and 

 (4) clastic or fragmental. The first of these (1), of which granite and 

 crystalline limestone are selected as types (Specimens Nos. 359G6 and 

 2G679), are composed wholly of crystals or crystalline granules without 

 trace of cementing material or glassy matter. Eocks of the second 

 group (2), of which obsidian is the type (Specimen 29631, from Mono 

 Lake, California), are made up wholly or in great part of amorphous 

 glass. This structure is confined to rocks of volcanic origin. Eocks of 

 this class pass by insensible gradations through microlitic, felsitic, and 

 micropegmatitic stages into holocrystalline forms. Eocks of the third 

 class (3), the collodial, are completely amorphous, and have a jelly or 

 glue- like structure. This structure is les 1 common than the others, and 

 is found only in rocks of chemical origin. It is illustrated by the silice- 

 ous sinter (Pealite, No. 28964) from the Yellowstone Park ; Semi Opal 

 from Louisiana (No. 38612) ; Flint nodule (No. 38012), from the chalk 

 cliffs of England ; and the green serpentine (No. 39038) from Mout- 

 ville, New Jersey. 



Eocks of the fourth group (4), of which sandstone is selected as the 

 type, are composed wholly of fragments of pre existing rocks, the indi- 

 vidual particles being held together by (1) cohesion, or (2) by a cement 

 composed of silica, iron oxides, carbonate of lime or clayey matter. A 

 coarse Triassic rock from Deerfield, Massachusetts (No. 26144), illus- 

 trates this type of structure. 



The exhibit is arranged as follows : 



1. Types of crystalline rocks : 



(I) Tourmaline granite, Minot, Maine. 35966 



(II) Crystalline limestone, Danville, New Jersey. 26679 



(1) Granular structure: 



Dolomite, Lee, Massachusetts. 27004 



(2) Compact structure : 



(a) Lithographic limestone, St. Louis, Missouri. 26719 



(b) Felsite, Marblehead Neck, Massachusetts. 35955 



